In preparing an image-bearing photosensitive surface, such as a pre-press proof or a photograph, it is frequently desirable to provide a protective covering to prevent or limit scratching, marring, or other damage to the image-bearing surface. Such a covering must be optically clear so as not to obscure or distort the image. It should also have flexibility, durability, and good adhesion to the image-bearing photosensitive element.
Items or objects that people use or see in their daily lives range in appearance from high gloss to dull or flat. Highly glossy surfaces reflect a high proportion of the light directed at them specularly. Gloss can be defined as the degree to which the finish of a surface approaches a perfect mirror. The closer the surface comes to approximating the front surface of a mirror, the higher the gloss. Gloss can be measured at many different angles, but the gloss of the surface increases with increasing angle of incidence from a line drawn perpendicular to the surface. Small angles of incidence, e.g., 20.degree., serve better to distinguish between examples exhibiting high gloss and are a better indication of gloss as perceived by the human eye. In measuring specular gloss, those skilled in the art use higher angle methods, e.g., 85.degree., to distinguish between lower gloss specimens and low angle methods, e.g., 20.degree., to distinguish between high gloss samples. In general, the polymeric films described in this patent fall into the high gloss category and the most meaningful measure of gloss is, therefore, the gloss measurement at 20.degree..
Since gloss significantly affects the appearance of an image, one of the most important requirements for an overcoat or protective covering for an image-bearing surface is the ability to achieve the desired degree of gloss. In many instances, it is desirable to modify the light reflectivity of a photograph or other imaged surface to reduce high gloss. In the photoimaging and image reproduction industry, it is desirable to modify the light reflectivity of the photosensitive materials which are used to prepare pre-press color proofs, and, more particularly, to be able to match the gloss of the finished proof to that of the ultimate press-printed product. High gloss protective coverings or coversheets on pre-press multilayer color proofs, which are designed to permit evaluation of color rendition before the actual printing run, interfere with the accurate assessment of color fidelity throughout the lithographic tone scale. This result is generally unacceptable to critical users of pre-press proofing methods.
The exact amount of gloss desired on a pre-press proof will vary depending on the type of end product whose appearance is to be duplicated, but in general, a level of 50-70 gloss units (as measured by a Gardner gloss meter at 20 degrees) is suitable for the commercial market in which the end product is a poster, brochure, or other print which is made on white, bright stock. On the other hand, there is a potential for great variability in the paper or other printing stock used in this segment of the industry and it may sometimes be necessary to obtain much lower gloss levels, e.g., a 20.degree. specular gloss lower than 25, when highly matte printing stock is to be used, prints made for the commercial market are generally printed with a sheet-fed press to individual, custom standards, which are not usually applicable to the entire industry. The ability to vary the degree of gloss on a pre-press proof aimed at this segment of the industry is frequently of great importance, and the instant invention is particularly useful for that purpose.
Lower gloss prints suitable for the publication market, in which the end product is a magazine print, are generally made on a more yellow, darker printing stock. The actual printing is frequently performed on a web press and the gloss is generally much lower, e.g., less than about 25 gloss units. In addition, the print may have a matte appearance to lower the gloss and provide contrast for easier reading of the periodical. In these cases, the determination of specular gloss may best be performed at a grazing angle, (e.g., 85 degrees) to better distinguish between lower gloss specimens.
Photosensitive materials used in color proofing are of the positive- or negative-working types. A positive-working process is described in U.S. Pat. No. 3,649,268 to Chu and Cohen. Positive-working materials may contain tacky, photohardenable compositions which, when exposed imagewise to actinic radiation, harden in the exposed image areas, resulting in tacky and complementary nontacky image areas. The image can be developed by applying colored particulate materials such as toners or pigments which selectively adhere to the unexposed tacky image areas.
A negative-working process is described in U.S. Pat. No. 4 174,216 to Cohen and Fan, which teaches a negative-working element having a support; a tacky, nonphotosensitive contiguous layer; a photohardenable photoadherent layer; and a strippable coversheet. After imagewise exposure to actinic radiation, the coversheet is peeled away, carrying with it the exposed areas of the photoadherent layer, and baring the tacky contiguous layer beneath. These areas of the tacky contiguous layer lying underneath the exposed image areas may then be toned with a finely divided particulate material. Different colored layers can be prepared and assembled in register over one another to form multilayer color proofs, as is well known to those skilled in the art. U.S. Pat. No. 4,053,313 to Fan describes a similar negative-working system which is developed by solvent washout.
Alternatively, a photosensitive imaging system may be precolored with dyes and/or pigments or other coloring materials, developed by washout, contain photoplasticizing agents instead of photohardening agents, and other variations known to those skilled in the art. Negative-working systems may be achieved in a number of different ways, for example, by utilizing a photosensitive layer which becomes tacky and tonable on exposure to actinic radiation. It is understood that in addition to various photopolymer elements, other nonsilver halide systems, e.g., diazo systems, are useful in preparing multicolor proofs. In addition the described protective polymer layers of this invention may be beneficially applied to many types of image bearing surfaces, e.g., a conventional silver halide photograph, where it is desired to provide a clear protective covering having a predetermined level of gloss. It is understood that the polymeric layers of the invention may also be applied to nonimaged glossy surfaces to obtain a desired level of lowered gloss.
Photosensitive elements have been provided with a great variety of protective coverings. U.S. Pat. No. 4,077,830 to Fulwiler (1978) teaches that photographic elements such as positives or negatives used for contact printing can be protected by application of a thin transparent sheet of resin material having a thin coating of a transparent pressure-sensitive adhesive. To avoid image distortion during printing, both the resin sheet and the adhesive layer must be extremely smooth. The protective resin sheet is preferably Mylar.RTM., a high gloss polyethylene terephthalate film manufactured by E. I. du Pont de Nemours and Company, Wilmington, Del.
Stone, U.S. Pat. No. 3,397,980 (1968) relates to a protective laminate comprising a durable outer layer such as Type D Mylar.RTM. and a polyethylene inner layer, bonded to microfilm using, a polyvinyl acetate coating. Type D Mylar.RTM. is very smooth and glossy.
Pre-press color proofs such as those described above are usually covered with one of three types of protective coverings or coversheets. The first of these is a positive-working photosensitive element as described in Chu and Cohen above, comprising a photosensitive layer and a polyester coversheet. This is exposed overall to actinic radiation and the polyester coversheet removed. This results in a proof with a desirable 20.degree. gloss level of about 65 gloss units. It suffers, however, from poor durability and is susceptible to crazing, scratching, and marring.
The second type of topcoat or coversheet is a negative-working photosensitive element as described in Cohen and Fan, above, which comprises a polyester coversheet; a photosensitive layer; and an organic contiguous layer. This is laminated to the proof and exposed overall with the polyester coversheet left in place. The result is a proof with a more durable finish but the gloss associated with the reflective, highly transparent integral polyester coversheet is unacceptably high, i.e., about 107 gloss units measured at 20.degree.. The unacceptability of such high gloss has long been recognized, and considerable effort has focused on ways to achieve a durable lower gloss protective coating that is resistant to crazing, scratching, marring, etc.
The third type of protective covering or coversheet is an integral layer of plastic film, such as polyester, coated with a nonphotoactive adhesive, and applied to the proof by lamination. The net result is indistinguishable from the second type of coversheet described above, in that it provides a durable covering, but one which exhibits unacceptably high gloss. For example, U.S. Pat. No. 4,329,420 to Bopp (1982) teaches a process of protecting pre-press proofs by applying a hard clear polymer layer by means of a pressure-sensitive adhesive. Bopp utilizes such high gloss materials as Mylar.RTM. polyethylene terephthalate.
A number of patents, however, teach methods of achieving a lower gloss protective coating for imaged surfaces. Many patents teach how to achieve coatings having a physically roughened or "matte" surface which diffuses light and has lowered gloss. For example. Spechler, U.S. Pat. No. 4,376,159 (1982), coats a transparent film-forming polymer on a matte-textured carrier sheet. The polymer is adhered to the surface of a color-proofing surprint with a pressure sensitive adhesive. The carrier sheet is then removed, leaving a matte-surfaced protective polymer coating on the surface of the proof.
King, U.S. Pat. No. 3,697,277 (1972) teaches a process for permanently bonding a protective coating to a photographic print. A matte-surface polyester film is placed on the wet emulsion surface of a photographic print and heat treated to permanently bond the film to the surface of the photograph.
Cohen and Fan, U.S. Pat. No. 4,286,046, recognizes the problems associated with the high gloss of a polyethylene terephthalate covering and teaches a method for delustering a photopolymer pre-press proof by applying particulate material to the tacky tonable layer.
A number of patents teach the use on photosensitive surfaces of matte layers, comprising a resin having dispersed therein insoluble particles or grains which impart a physical roughness to the surface. For example, U.S. Pat. No. 4,168,979 to Okishi et al., and U.S. Pat. No. 4,238,560, to Nakamura et al., teach a light sensitive printing plate with a matte overlayer, which is a resin layer with an insoluble particulate matting agent dispersed therein. This achieves a surface coarseness or roughness which contributes to improved exposure by the vacuum contacting method. After exposure the matte overlayer is removed with the developer. Similarly, U.S. Pat. No. 4,235,959, to Thijs, deceased, et al., teaches a photosensitive silver halide layer incorporating a layer comprising gelatin and dispersed spherical particles which are the polycondensation product of urea, formaldehyde, and silica particles.
Although compatibility and incompatibility of polymers are recognized phenomena, the prior art does not disclose integral films prepared from blends of slightly incompatible polymers, nor does it teach how such films can be laminated to imaged surfaces to provide a controlled degree of lowered gloss. On the contrary, in general the prior art teaches that incompatibility is undesirable and to be avoided.
U.S. Pat. No. 3,671,236, to Van Beusekom, teaches that in preparing a proof by superimposing a plurality of colored sheets, the individual sheets must be perfectly colorless and optically transparent in order to avoid any "haze" or imperfection being multiplied in the several sheets.
U.S. Pat. No. 4,323,636 to Chen teaches that in order to obtain a transparent, i.e., nonlightscattering, photosensitive composition for preparing printing plates, the binder and the monomer must be compatible. Incompatibility is evidenced by the formation of haze in the photosensitive composition. By compatibility is meant the ability of the constituents to remain dispersed with one another without causing appreciable scattering of light. This occurs when the monomer is soluble in either of the component blocks of the block copolymer. Chen teaches only how to avoid incompatibility of polymer and monomer in photopolymerizable compositions. It does not deal with compatibility/incompatibility of blends of polymers, nor does it suggest any utility for such incompatibility.
Gallagher et al., U.S. Pat. No. 3,811,924 (1974) teaches that an aqueous lacquer coating composition which is a blend of polymers consisting essentially of an ammonium salt of a copolymer of ethyl methacrylate and methacrylic acid together with poly(vinyl acetate) can be applied to glossy surfaces to provide a degree of roughness or "matte". Gallagher also teaches, however, that otherwise similar non-ammonium polymer mixtures laid down from an organic solvent system would lack the desirable matte finish. The polymeric films of the invention are not ammonium salts, and they are coated on a substrate from an essentially organic solvent system to form a smooth, low gloss integral film prior to laminating to the image-bearing surface.